原理：点击图片蒙版层出现，显示图片大图，再次点击蒙版层隐藏。

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>原生js点击图片放大效果简单实现</title>
    <meta name="renderer" content="webkit">
    <style>
        .imgPreview {
            display: none;
            top: 0;
            width: 100%;
            height: 100%;
            position: fixed;
            background: rgba(0, 0, 0, 0.5);
        }
        .imgPreview img {
            z-index: 100;
            width: 90%;
            height: auto;
            position: fixed;
            top: 50%;
            transform: translate(-50%, -50%);
            left: 50%;
        }
    </style>
</head>
<body>
    <div>
        <!-- 给需要的该效果的img图片添加class="img" -->
        <img src="https://cdn.jsdelivr.net/gh/wuzhiguang1/bolgfile@2.0/photo/4ed4255c5172b148a5780260e48ee01a--59873749.jpg" 
        class="img" style="width: 300px;"> 
        

    </div>
    <!-- 添加蒙版层 -->
    <div class="imgPreview">
        <img src="#" alt="" id="imgPreview">
    </div>
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.js"></script>
    <script>
        $('.img').on('click', function () {
            var src = $(this).attr('src');
            $('.imgPreview img').attr('src', src);
            $('.imgPreview').show()
        });
        $('.imgPreview').on('click', function () {
            $('.imgPreview').hide()
        });
    </script>
</body>
</html>

效果图如下：